In brazing applications, the phenomenon known as ‘Liquid Film Migration’ or LFM, causes a deterioration in the overall performance of brazed products such as evaporators, radiators, heater cores etc. In literature the term “LFM” is also referred to as “core dissolution” or “core penetration” or “core erosion”. Herein by the term “LFM” we refer to all these terminologies. Although the exact mechanism causing LFM is not yet fully understood, it appears that the severity of LFM is enhanced by the presence of a certain amount of dislocations in the core alloy of the brazing sheet. It is known that the sensitivity of a material to LFM is relatively low in both, fully annealed (O-temper) and in strain hardened and/or stress relieved tempers (such as for example H14, H24 etc) as compared to the soft and slightly cold worked condition of the same material. By the term “slight cold working”, we refer to the deformation resulting from industrial processes such as stamping, roll forming or tension levelling which are typically applied to produce components of heat exchangers such as evaporator or oil cooler core plates, folded tubes etc. When a brazing sheet consisting of a core alloy and an Al—Si clad alloy is deformed to form a product and is subsequently subjected to a brazing cycle, the small amount of deformation appears to be sufficient to induce LFM in the brazing sheet. If the LFM progresses too far into the core alloy, then the brazeability, strength and the corrosion resistance decreases. It is known that alloying elements, which retard recrystallisation, such as chromium, zirconium and vanadium enhance the susceptibility to LFM. Manganese dispersoïds are also known to retard recrystallisation and therefore to enhance the susceptibility to LFM. The amount and size of the manganese dispersoïds depend on the processing route of the brazing sheet.
For brazing applications, a core alloy of a brazing sheet product requires a good combination of strength and formability. Obviously, the susceptibility to LFM has to be at a sufficiently low level to ensure adequate corrosion resistance and brazeability. Higher strength can be obtained by alloying with elements such as silicon, manganese, chromium, zirconium or vanadium. However, these alloying elements also increase the susceptibility to LFM. The use of a non O-temper, such as H14-temper or H24-temper has also been suggested to reduce the susceptibility to LFM. However, although these tempers effectively reduce the LFM, formability of the brazing sheet product is often compromised. Other alternative processes such light cold deforming process such as tension levelling, or the use of a non-recrystallised surface layer are difficult to control in mass-production practice and therefore may compromise reproducibility and/or formability.